[libc][NFC] Add noreturn and constexpr qualifiers where appropriate

These functions make it clear to the compiler and user what the intended
behavior is so llvm can make them go as fast as possible.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D106807

GitOrigin-RevId: 640ed21cceb28ddb3a9779fdfa11a4c4d97e29df
3 files changed